35S 



Aliiskti — Our Living Rt'simices 



For further information: 



Kurt Jenkins 



National Biological Service 



Wrangell-St, Elias Field Station 



PO Box 29 



Glennallen, AK 99588 



during the peak nf calving; observers looked for 

 presence ot calves or swollen udders, indicating 

 cows had or would soon produce calves. 



In 1993 biologists measured survival rates of 

 calves and adult cows to help interpret causes of 

 the rapid population decline observed in the 

 early 199()"s. They measured survival rates by 

 fitting 39 calves and 41 adult cows with radio- 

 collars containing mortality sensors. They 

 located these radio-collared caribou daily dur- 

 ing the calving season in 1993. weekly during 

 the remainder of summer, and once every 2 

 months throughout winter. When biologists 

 located carcasses of dead caribou, they inspect- 

 ed them as soon as possible to determine the 

 cause of death. 



Population Trends 



The Mentasta herd increased from about 

 2.000 caribou in the early 1970's to 3,200 in the 

 early 1980"s. an increase of about 5% per year 

 (Fig. I). Since 1989 the Mentasta herd has 

 decreased to a low of around 900 caribou in 

 1993, a decrease of about 24% per year. 

 Between 1992 and 1993 alone, the herd 

 decreased by one-third. 



This population decline appears related, in 

 part, to changes in calf survival or production 

 between the I980"s and 199()"s. The proportion 

 of cows with calves in late June declined from 

 39 calves to every 100 cows in the late I970's 

 and early 1980's (including a high of about 50 

 calves to 100 cows in 1979), to only 6 calves to 

 every 100 cows in the early 1990"s (Fig. 2). 

 Similarly, estimates of calf-to-cow ratios in 

 October have decreased about 90% since the 

 1980"s. 



Recent surveys of birth rates indicate that 

 rapid declines in calf-to-cow ratios were not 

 related to poor productivity of cows. In 1992 an 

 estimated 81% of cows produced calves: in 

 1993, 70% did. Although birth rates were below 

 average in 1993, producti\ity was sufficient for 

 the herd to grow in the absence of high calf loss- 

 es. 



By intensively radio tracking newborn 

 calves, biologists showed that the low calf-to- 

 cow ratios were related to high death rates of 

 calves. Of 39 calves radio-collared at birth, only 

 1 (2.5%) survived the summer. The rest were 

 lost to predation by gray wolves (Canis lupus). 

 grizzly bears (Ursus arctos), and wolverines 

 (Gulo gulo), or they died from unknown causes. 



3.500 



500- 



73 75 77 79 



83 85 87 89 91 93 

 Year 



Fig. 1. Recent trends in size of the Mentasta caribou 

 herd. 197.1-4.V 



73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 

 Year 



Fig. 2. Recent trends in calf production (June) and num- 

 ber of calves (October) in the Mentasta canbou herd, 

 1973-93. 



Survival rates of adult cows also were low. 

 Of 41 cows radio-collared at the beginning of 

 the study, only 83% (34 cows) survived 1 year. 

 By contrast, generally 88%-96% of adult cows 

 survive each year in stable or increasing herds. 



Ongoing monitoring will increase under- 

 standing of natural tluctuations of the herd and 

 provide information for incorporating fluctua- 

 tions into a scheme for determining harvest 

 quotas. Currently, biologists propose to allow 

 annual harvests equal to a small percentage of 

 the number of calves in the herd each fall, a 

 good index of population trends. This proposal 

 will link the harvest to patterns of herd growth 

 and incorporate the objectives of natural popu- 

 lations and resource use into one workable man- 

 agement model. 



